Process for producing dispersion-modified alloys of chromium and iron-group metals



3,425,822 PROCESS FOR PRODUCING DISPERSION-MODI- FIED ALLOYS F CHRGMIUMAND IRON- GROUP METALS John B. Lambert, Mill Creek Hundred, and John T.Looby, Christiana Hundred, DeL, assignors, by meSne assignments, toFansteel Metallurgical Corporation, a corporation of New York NoDrawing. Filed Apr. 19, 1966, Ser. No. 543,519 US. Cl. 75.5 8 ClaimsInt. Cl. C22c 1/04, 1/06, 19/00 This invention relates to an improvementin processes for making dispersion-modified alloys of chromium andiron-group metals. More particularly the invention is directed toprocesses for making a metal composition containing 0.5 to 40% by weightof chromium, in which processes (1) a mixture comprising (a) an oxide ofa metal selected from the group consisting of iron, cobalt and nickel,(b) a particulate refractory oxide having a free energy of formation at1000 C. greater than 103 kilocalories per gram atom of oxygen and aparticle size below about 35 millimicrons, and (c) an oxide of chromium,is prepared and (2) oxides (a) and (c) are deoxidized by heating themixture with a carbonaceous reducing agent until the oxygen content ofthe product in excess of that combined in oxide (b) is below about 2000p.p.m., the invention being in the improvement which comprises (3)decarburizing the deoxidized mixture by heating at a temperature in therange of 700 to 1000 C. in a flowing gas stream containing hydrogen, thepressure of hydrogen being at least two atmospheres, the decarburizingbeing continued at least until substantially all elemental carbon hasbeen removed.

The preparation, by powder metallurgy methods, of alloys containingchromium and iron-group metals, which alloys are dispersion-strengthenedwith particulate refractory oxides such as thoria, has been described inUS. Patent 2,972,529, issued Feb. 21, 1961, to Alexander, Iler and West.This patent describes a method wherein the hydrous oxides of aniron-group metal and of chromium are coprecipitated with the reinforcingoxide particles, the precipitate is dried, and the chromium andiron-group metal oxides are chemically reduced to the correspondingmetals containing the non-reduced reinforcing oxide particles in adispersed condition. This chemical reduction is effected at elevatedtemperatures, as high as 1100 C., using hydrogen or various carbonaceousagents, for such prolonged times as eighty hours. The reduced powder isconsolidated into solid metal form by known powder metallurgytechniques.

In processes such as above described, the size of the reinforcingrefractory oxide particles, hereinafter sometimes called fillerparticles, is critical. The particles must be and remain sub-micron insize--that is, have an average size below 1 micron, and preferably theyhave a size less than 0.5 micron. Unfortunately the oxide fillerparticles have a marked tendency to grow during the early stages ofthese processes, if they are exposed to high temperature. This dangerexists primarily while the matrix metal phase is in powder form andbefore it has been consolidated to near maximum density. Such particlegrowth is particularly troublesome when the desired filler particle sizeis not over 100 mg, a much preferred limit.

Since time is also a factor in particle growth, the chemical reductionof the matrix metal oxides should be carried out at as low a temperatureand in as short a time as possible. However, although hydrogen willreduce oxides of nickel and cobalt in a reasonable time at-say, 750 C.,the reduction of chromium oxide with hydrogen requires temperaturesabove 1100 C. and considerably longer times.

Carbonaceous reducing agents, such as powdered carnited States Patent 0bon and hydrocarbon gas, have been found capable of deoxidizing Cr Oeven below 1100 C. This alleviated the oxide filler growth problem butintroduced a further problem-namely, the leaving of a residue of carbon.Chromium is a moderately strong carbide former, so when chromium is tobe a constituent of the alloy formed the corresponding chromium carbideresidues present problems of removal, although sometimes they may beleft as a second dispersed phase. Because the carbides generally havegreater solubility in the matrix than the carbides, their absence ispreferred for highest use temperatures or use in an oxidizingenvironment.

It has been proposed to solve this problem of residual carbon by puttingonly the stoichiometric amount of carbon into the oxide mixture. Whilethis avoids the excess free carbon, reducible metal oxides or some ofthe carbides are likely to remain. The oxide in such reducible oxides isherein called excess oxygen. This term includes all oxygen present otherthan that combined as the filler oxide. Such oxygen is not desired inthe product because it leads to lack of stability and may even act as aflux causing growth of the filler particles. Generally, in processesinvolving stoichiometric amounts of solid reactants the attainment offinal equilibrium is slow and hence, in this field, a product low inboth residual carbon and excess oxygen is difiicult to obtain. Itinvolves a lengthy endpointing procedure at elevated temperature, whichin turn jeopardizes the preservation of the smaller filler particles.

The use of hydrocarbon gases for deoxidizing such chromeoxide-containing metal oxide mixtures has been tried. The temperaturesrequired, e.g. around 700 C.- 900 C., cause cracking of these gases andthe resulting carbon deposits itself preferentially on the surface ofthe mixed oxide charge thereby blocking diffusion of the reagents intothe interior of the mass. Again, the problems of very long time cyclesand excess residual carbon appear.

Now according to the present invention it has been found that if amixture comprising (a) an iron-group metal oxide, (b) particles of arefractory oxide having a free energy of formation at 1000 C. greaterthan 103 kilocalories per gram atom of oxygen and an average particlesize below about 35 millimicrons, and (c) chromium oxide is .preparedand oxides (a) and (c) are deoxidized by heating the mixture with acarbonaceous reducing agent until the oxygen content of oxides (a) and(c), combined, is below about 2000 parts per million, and the so-reducedproduct is decarburized by heating it at a temperature in the range of700 to 1000 C. in a flowing gas stream containing hydrogen, the pressureof hydrogen being at least two atmospheres, the chromium and iron-groupmetal oxides can be substantially completely deoxidized and the productdecarburized to the extent desired without objectionable growth of therefractory oxide particles. In describing the invention, the separateterms deoxidation and decarburization are used in preference toreduction to distinguish more clearly between the first step, i.e. ofremoving oxygen from the matrix metal oxides, and the second step, i.e.of removing carbon from the deoxidized product. It could be argued thatboth steps are reducing in nature. In products produced according to theinvention the refractory oxide is sometimes referred to as the dispersedphase and the metal formed by reduction of the iron-group metal oxide,the chromium oxide, and any similar, optional, reducible metal oxides issometimes called the matrix metal.

The iron-group metal oxide of the starting mixture can be an oxide ofiron, cobalt or nickel or of mixtures of any two or all three of thesemetals. The proportion of chromium oxide in the starting mixture issufiicient to give about from 0.5 to 40% by weight of chromium metal inthe product after deoxidation and decarburization. Optionally there mayalso be present oxides of such metals as tungsten, molybdenum, manganeseand vanadium. The metal oxides are not necessarily completely anhydrousat the start, but if not, the mixture may be given a low temperature(e.g. 450 C.) calcination, since moisture interferes with thedeoxidation and makes quantitative reduction difficult if not impossibleto achieve.

The particulate refractory oxide, dispersed in the mixture of oxides ofthe matrix metals, must be an oxide having a free energy of formation(sometimes designated as negative), at 1000 C., greater than 103kilocalories per gram atom of oxygen. This group includes the oxides ofaluminum, cerium, hafnium, uranium, magnesium, thorium, beryllium,lanthanum, calcium, and yttrium. The refractory oxide must be extremelyfinely divided, having an average particle size less than about 35millimicrons. The proportion of refractory oxide present should be suchas to give about from .05 to 20 percent by volume in the final product.Intimate dispersion of the refractory oxide particles in the matrixmetal oxides can be achieved by such methods as coprecipitating it froma colloidal sol or coprecipitating its precursor in the form of anoxygencontaining compound of the metal and converting such precursor tothe oxide of the metal, as by low temperature calcination. For instance,the presence of dispersed, finely divided thoria can be effected byprecipitating it from a thoria sol, or by precipitating thorium oxalateand converting this thermally to thoria.

The starting mixture of oxides constituted as above described isdeoxidized by heating it with a carbonaceous reducing agent until theexcess oxygen content of the mixture is below about 2000 parts permillion in the deoxidized product. The carbonaceous reducing agent canbe any carbon-containing material or compound which upon contact withthe reducible metal oxides will react with oxygen therein and remove it.Elemental carbon, carbon monoxide, or hydrocarbons such as methane, areamong the preferred agents which can be used. Other reducing, i.e.,deoxidizing, agents such as hydrogen, as Well as inert gases such asargon, neon, or krypton can be used in combination with the carbonaceousagent. The irongroup metal oxides, for example, are quite rapidlyreduced by hydrogen alone, and a preferred procedure sometimes is toinitially reduce with hydrogen and then complete the deoxidation byintroducing the carbonaceous agent, such as methane, either alone or incombination with the continued presence of hydrogen. When gaseousreducing agents, such as hydrogen-methane mixtures, are used, they mustmake penetrating contact with the oxides being deoxidized. One effectiveway to effect such contact is to dispose the oxides as a thin layerhaving a thickness less than about 6 millimeters, and pass the gasstream in contact with such layer.

In one preferred aspect, the chromium oxide is reduced using acombination of powdered carbon and methane as reducing agents. In thisinstance, with any practical thickness of charge, there is a limitationon the availability of methane to the inner zones of the charge, basedon the diffusion characteristics. Inclusion of powdered carbonthroughout the oxide charge not only overcomes this limitation in zoneswhere methane is scarce, but also enhances the rate of deoxidationthroughout. The carbon should be of high purity with sulfur content notover 200 ppm. and preferably less than 100. Its ash content should beless than 0.10%, and its surface area should be in excess of 20 m pergram. The preferred carbon is amorphous, rather than graphitic, by X-raycharacterization.

If methane is used as a reducing agent, the partial pressure of this gasfed into contact with the oxide should be controlled so as to avoidformation of excessive carbon by cracking. Satisfactory control isachieved using mixtures of hydrogen and methane when the partialpressure of the methane is held within limits defined by the expression:

Partial pressure of OH; in atmospheres: P 2

where p is the sum of the partial pressures of hydrogen and methane inatmospheres, T is the temperature in the deoxidation zone in degreesKelvin, and F is a number ranging from 0.73 to 1.2. The value of 1 mayrange from Zero to slightly above ambient pressure, and other gases,e.g., argon, may be used, if desired, to dilute the methane hydrogenmixture. T is in the range of 998 K. to 1323 K. When 12 in the foregoingexpression is zero, the alter nate condition of deoxidizing withelemental carbon in a vacuum or flowing inert gas is in effect.

In a typical instance, 1 will be 1.5 atmospheres, and T will be 1175 K.In applying the formula, the desired methane partial pressure incalculated to be 0.031 to 0.050 atmosphere, the limits of the rangebeing defined by corresponding values of the factor, F.

The heating during the deoxidation step should be sufficient to completethe reduciton of the matrix metal oxides in a reasonable length of time,but at the same time the temperature should not be high enough to causesubstantial growth or agglomeration of the dispersed refractory oxideparticles. Temperatures in the range of 925 to 1025 C. have been foundto be quite satisfactory, and somewhat higher temperatures can be used.Some of the considerations involved in the deoxidation step arediscussed in the above-mentioned Alexander, Iler and West US. Patent2,972,529.

The improvement of the present invention lies in the combination of anovel decarburization process with the deoxidation processes abovedescribed. This decarbur'mation process or step comprises heating thedeoxidized product at a temperature below 1000 C. in a flowing streamcontaining hydrogen under at least two atmospheres partial pressureuntil at least the elemental carbon content of the product issubstantially removed. To accomplish the desired degree ofdecarburization in a reasonable time the temperature during this stepshould be above about 700 C.; temperatures in the range of about 700 to800 C. are preferred. An especially effective heating cycle is to carryout the decarburization substantially in the range of 700 to 800 C. andthereafter complete it in the range of 825 to 950 C.

The flowing hydrogen in the decarburizing step must make penetratingcontact with the product which is being decarburized. Such contact is,of course, facilitated by having the hydrogen under at least twoatmospheres partial pressure, but it can be facilitated in other waysalso. High velocity of hydrogen flow increases the rate of carbonremoval, but, if it is desired to obtain the maximum pick-up of carbonper pass, the velocity of flow can be retarded. If the product iscontained in trays during decarburization the layers of product can berelatively thin. Prior to initiating decarburization the deoxidizedpowder can be formed into porous pellets, into and around which thehydrogen can flow with maximum ease, and such pellets can be supportedon screens. Any of these methods or any combination of them can be usedto insure penetrating contact by the hydrogen.

The carbon removed from the product at this stage passes off primarilyin the form of methane. It will be evident that measures which assurepenetrating contact by the flowing hydrogen at the same time facilitateremoval of the methane formed.

The hydrogen used for decarburizing must be dry, because any moisturepresent in it will tend to reoxidize the metals present and cause thereducible oxide content to be unacceptably high. Methods for dryinghydrogen are well known in the art.

Irrespective of the manner in which the deoxidation with thecarbonaceous reducing agent is carried out, carbon is present after thedeoxidation, either as metal carbides, free carbon, or both. It has beendiscovered that the herein-described decarburization, which includes thereaction between hydrogen and both the free carbon and the carbidecarbon to produce methane, is promoted, apparently catalytically, by theintimate presence of nickel or cobalt metal. This is particularly truewhen these metals are formed in situ during the deoxidation step. Whilenickel is well known to be a catalyst for the hydrogenation of fluidsubstances, it is surprising that the reaction with solid carbon orcarbides would be thus affected. The fortuitious cooperation between theeffect of these essential components and the use of hydrogen atsuperatmospheric pressure results in such marked improvement in thisstep that advantageous quality characteristics appear in the finalproduct.

The decarburization step has been found susceptible to improvement byadditional temperature control. At the beginning of the decarburizationstep the reaction mass usually contains residual free carbon as well ascarbides. The free carbon appears to be converted to methane mostrapidly in this environment between about 700 and 800 C., while thecarbides react more rapidly in the 825 to 1000 C. range. Therefore, thedecarburization process is preferably operated at about 750 C. until themethane evolution slows substantially and then the temperature is raisedto the 825 C. to 1000 C. range to complete reaction with the carbide.The most preferred low range is 725-800 C. and the high range 825950 C.,with carbon removal to less than 1000 ppm.

This dual temperature range method is particularly valuable when a largeexcess of free carbon is present. Such excess may be deliberately usedto speed up and complete the deoxidation step, or when inadvertentdeposits of car-hon occur due to loss of process control with respect tomethane cracking. Furthermore, if carbide particles are to be left as asecond dispersed phase, the decarburization at the low temperatureoffers excellent means for preserving the carbides. Also, the amount ofcarbide left can be further limited in a controlled manner by partialdecarburization in the higher temperature range.

The deoxidation and decarburization procedures of this invention can beapplied both to loose powder charges and to compacted powders which havea density not more than about 85% of theoretical and which are,therefore, porous enough for the penetrating passage of the gases, byflow or diffusion, through the mass. In either case the charge of mixedoxides can be treated in a variety of vessels or containers which permitthe penetrating contact of the reactive gases. The powder or compact canbe enclosed in a suitable metal container or sheath, through which thedeoxidizing and decarburizing atmospheres are passed and the finishedmetal can be processrolled, extruded or otherwise worked by known meanswhile still in the sheath to avoid atmospheric contamination.

In one particular embodiment, for example the invention was applied to acompact of the reduced product, i.e., a porous billet of nickel-chromiumalloy containing thoria, which had previously been reduced while inpowder form but which during handling had been slightly contaminatedwith adsorbed oxygen. This oxygen was believed to be present as chromiumoxide and its removal was considered necessary. This could be doneaccording to this invention by mixing the contaminated powder with finecarbon, partially compacting the mixture, and enclosing the compact in asheet-metal sheath provided at the ends with gas inlet and outlet tubes.The excess oxygen could then be removed by passing the hydrogen andmethane through the sheath and following the steps and conditions setforth above to complete the deoxidation and decarburization. The excessoxygen and free carbon could thus be substantially completely removed,and the purified porous metal billet could be worked by suchcompression-working procedures as extrusion or forging,

6 while still in the sheath, to density the billet to the point wherethe total surface area is so reduced that contamination by gaseousadsorption, when the billet is removed from the sheath, is no longersignificant.

While the improvements of this invention are directed primarily at thechemical processing of the oxides of the matrix metals Co, Ni, V, Cr,Fe, Mn, Mo and W, it should be understood that the invention may bepracticed successfully in the presence of other alloying metals whichmay be used in minor amounts to modify the product. Such ancillarymetals include copper, silver, bismuth, cadium, tin, platinum and otherswhich are deoxidized and decarburized under the conditions of theinstant process. These metals and others, such as titanium, zirconium,hafnium, aluminum, columbium, tantalum, rhenium, and beryllium can beadded as metal powders to the deoxidized and decarburized powderedproducts made according to this invention, prior to known consolidationand working steps. In general, these ancillary metals are used only inminor amounts.

Although the two essential steps of deoxidation and decarburization aretaken in sequence, it is not always necessary to complete thedeoxidation step before proceeding to the decarburizing step. Forexample, when or more of the available oxygen has been removed by thefirst step, the pressurized hydrogen atmosphere and elevated temperaturecondition can be applied and decarburization started. There is enoughdeoxidizing potential in the residual carbon and carbides, acting inpart through the methane generated in situ, to remove the last portionof the excess oxygen.

The utility of this invention can be expressed in terms of theadvantages of the claimed processes as follows:

(1) The substantially complete removal of oxygen from reducible oxidesfrom the described metal oxide mixtures.

(2) The substantially complete removal of free carbon as well as thechoice of removing, in addition, all the combined carbon or leavingdesired portions of the metal carbides in the product as a seconddispersion-modifying phase.

(3) The achievement of the above two advantages at lower temperaturesand shorter times than heretofore practiced, thereby realizing theimportant advantage of preventing undesired growth of thesubmicron-sized oxide filler particles.

(4) The achievement of improved quality characteristics in the resultingproducts, such as higher integrity, improved high temperature strength,and better workability because of the very low content of excess oxygenand free carbon and the preservation of the filler particle fineness.

(5) The realization of economic advantages accompanying the technicaladvantages with respect to use of lower temperatures and shorter timecycles.

(6) The advantageous control, in a preferred modification of the novalprocesses, relating to efiicient use of fine carbon as a deoxidizingagent as well as in providing a regulated carbide dispersed phase in theproduct.

The invention will be better understood by reference to the followingillustrative examples:

Example 1 This example descripes the deoxidation and decarburization ofa mixture of Cr O MO and ThO prepared by thermal decomposition of amixture of the corresponding salts.

A mixture of parts of Cr(NO -9H O, 470 parts of Ni(NO -6H O, and 5 partsof Th(NO -4H O was heated to approximately 90 C., at which temperaturethe mixture was a molten, homogeneous liquid. The solution was fed to atwo-fluid, pneumatic atomizer which injected droplets of the liquid intoa cocurrent hot air stream which discharged into a spray chamber. Thedrying air admitted to the spray chamber was preheated to 700725 C., andthe flow :rate of this air was maintained so that the gas temperature atthe chamber exit was above 375 C. The atomized mixture in the chamberformed solid, mixed oxide particles which were collected in cyclones.The powder collected was then given a final two-hour heat treatment inan oven at 450 C. to convert any residual nitrate salts to thecorresponding oxides.

The resulting oxide powder, analyzing by weight 76.4% NiO, 21.8% Cr Oand 1.7% ThO was blended with carbon black in a twin-shell blender for 2hours. This carbon black powder had the following characteristics: Bulkdensity 0.31 g./cc., surface area 117 m. /gm., oxygen 2.4%, nitrogenless than 0.15%, hydrogen 0.45%, sulfur less than 100 p.p.m., ash0.008%. An electron micrograph indicated that the carbon black was inthe form of beads 2535 millimicrons in diameter. X-ray indicated it tobe mostly amorphous. The previously dried and pulverized carbon blackwas mixed in the ratio of 6 parts by weight of carbon per 100 parts ofmixed oxide.

After blending, the material was placed in fiat trays to a 1-inch depth.A stack of these trays was then placed in a reactor provided with a dryhydrogen atmosphere, heated at slightly above atmospheric pressure to500 C., and held for 4 hours to reduce the NiO portion of the oxidemixture. Only a negligible fraction of the carbon was consumed duringthe foregoing step. The reactor was then purged and heated under argonto 950 C.

When the reactor temperature reached 950 C., the argon flow was stoppedand a methane-hydrogen mixture at a pressure of 1.3 atmospheres andanalyzing 2 volume percent methane, was introduced at a. flow rate ofabout 30 linear feet per minute across the powder for seven hours.

The methane flow was discontinued and the oxide reduction was finishedusing a flow of pure hydrogen for two hours at 950 C. and a pressure ofabout atmospheric. The hydrogen flow was then continued 14 hours at 850C. and approximately two atmospheres pressure to decarburize the reducedmixture, and the charge was then cooled.

The product was a loosely-sintered powder containing nickel and chromiumin a 4:1 weight ratio, and having 2% by volume of submicron-sized thoriaparticles well dispersed therein. Oxygen analyses of this materialshowed it to contain about 1000 p.p.m. oxygen in excess of the oxygencombined as T110 and to have a carbon content less than 0.03%. The ThOcrystallite size was determined to be 17 millimicrons.

Example 2 This example illustrates the application of this invention tothe treatment of a metal oxide-contaminated powder.

A 1200 gm. portion of a nickel-chromium-thoria alloy powder prepared bya coprecipitation method and containing 99 p.p.m. carbon and 2300 p.p.m.oxygen in excess of that present as ThO was ground to pass a 30-meshscreen and then compacted hydrostatically with 50,000 psi. pressure toform a billet about 2 inches in diameter. The billet was machined andencased in a close-fitting 316 stainless steel cylinder capped at theends. A tubing line was attached to each end to allow gas flow throughthe compact. The cylinder containing the billet was placed in a furnace,and the tubing on one end was connected to a gas supply manifold; theother end was vented.

Hydrogen was passed through the billet at a rate of 2.8 liters perminute. The temperature of the billet was raised slowly in astepwise-mannerthat is, the temperature was held 1 hour at 200 C., hourat 300 C., and 2 hours at 450 C. before being elevated to 1000 C. At thelatter temperature, methane was mixed with hydrogen to give 1% by volumeCH in the gas feed. The methane flow was stopped after two hours and thetemperature held an additional hour under pure hydrogen.

The temperature was thereafter lowered to 800 C. for decarburization,and the H inlet pressure raised to 3 atmospheres and the hydrogen flowcontinued 2 hours at this temperature. This removed substantially allcarbon from the product. The billet temperature was then raised to 1000C. and held an additional hour at this temperature, and the billet wasthen cooled. The cooled billet was purged with pure argon, and theconnecting tube pinched off to prevent exposure to air.

The billet was next heated to 1093 C. and extruded to a A x 1"rectangular bar. The stainless steel covering was removed. The metalcontained a negligible quantity of oxygen in excess of that present asthoria and a residual carbon less than 50 parts per million.Metallographic examination at a magnification of times revealed amicro-structure with exceptionally few defects attributable to particlesof oxide or carbide. This bar was suitable for reduction to sheet havingdesirable strength, oxidation resistance, and stability above 1093 C.

Example 3 This example describes the preparation of a nickel-base alloycomposition containing approximately 20% chromium and 0.5% manganese andhaving 2% ThO dispersed therein, A coprecipitate of nickel, chromium andthorium hydroxycarbonates was prepared by neutralizing the correspondingnitrate salts, in solution, with ammonium carbonate. After filtering andwashing, the wet cake was reslurried in a tank with about 25 liters ofdemineralized water. The pH of the slurry was found to be 8.1. One literof an aqueous solution containing 219 gm. of dissolved Mn(NO was thenadded to the well-stirred slurry over a one-half hour period. The finalpH was 7.9. The resultant mixture was again filtered but not washed, andthe cake was dried, calcined, and ground and blended as in Example 4.The weight of the mixed oxide product was 19.4 pounds.

To deoxidize this oxide, 9.7 pounds of it were mixed for two hours in atwin-shell blender with 1.16 pounds of finely-divided carbon black, andthe mixture was filled into fiat-bottom trays to a depth of /1 inch. Thetrays were stacked inside a furnace, and the furnace was supplied withhydrogen at 1.2 atmospheres pressure, heated to 400 C., and held sixhours at this temperature. The hydrogen flow over the trays wasmaintained at an average linear rate of about 30 feet per minute.

The furnace was then heated at a rate of C. per hour to 925950 C., theflow rate and pressure being held constant. Methane was admitted at thebeginning of this heatup. The partial pressure of methane was adjustedto 0.023 atmospheres, as indicated by analysis of the inlet gas. Afterholding 29.5 hours at 925 950 C. the methane flow was stopped and thetemperature was elevated to 1025 C. for one hour.

The reactor was then cooled to 800-825 C. in preparation for thedecarburization step. The hydrogen partial pressure was raised to about2 atmospheres and hydrogen flow through the reactor was continued. Thesample was thus essentially decarburized, as indicated by absence ofmethane in the reactor elfiuent gas after 60 hours. No significantincrease in methane evolution occurred when the reactor was heated toabout 925 C., at which temperature it was held /2 hour before cooling.

The product recovered weighed 6.8 pounds and analyzed as follows:

The material was suitable for further processing by standard powdermetallurgy techniques to give useful metal articles either as bar, sheetor tubing.

Example 4 In this example, a mixed oxide powder was prepared by aco-precipitation process. Three fluids, designated Fluids A, B, and C,were fed simultaneously through inlet Ts into a recirculation line, theapparatus for which was similar to that of Example 1 of US. 2,972,529.Fluid A consisted of 68.4 pounds of Ni(NO -6H O, 25.3 pounds of Cr(NO)-9H O and 0.75 pound of dissolved in demineralized water and diluted to40- liters. Fluid B was approximately a 3.2 molar (NH CO solution. FluidC was demineralized water. Three liters of water were put in theapparatus to fill the lines and prime the circulating pump. Fluids A andC were fed at equal rates into the recirculation line over a two-hourperiod, and Fluid B was fed so as to control the pH of the slurry beingrecirculated at 7.0. At the end of the precipitation, the slurry wasfiltered on a plate-and-frame press and washed with 330 liters ofdemineralized water.

An eight-hundred gram charge of the filter cake obtained from thiscoprecipitation was reslu rried in 2 liters of deionized water, and a24-gram portion of the same carbon black described in Example 1 wasadded. Several drops of a non-ionic Wetting agent were also added tofacilitate the wetting of the carbon black. Vigorous agitation wasemployed to assure thorough mixing, after which, the charge was filteredand dried at 125 C. The dried cake was placed in a vacuum oven andheated for 4 hours under a vacuum of about 5 mm. Hg at 475 C. Analysisof the oxide obtained after this calcining showed it to contain 6.16%carbon. Additional carbon was then added to the calcined oxide by a.ball-milling process. An additional 5.45 gram portion of carbon blackwas blended with 100 grams of the calcined oxide; the mixture was thenball-milled for 16 hours. The product was screened to pass 100 mesh andenough water was stirred in to form a thick paste. Cylindrical pelletsapproximately 4 inch in diameter and inch high, were made by squeezingthe paste through a suitable mold. After drying, the pellets werecharged to a cylindrical reactor to give a 2-inch bed depth.

Hydrogen gas was admitted to the reactor and passed through the bed ofpellets at a velocity of approximately 6 ft./second at standardconditions. The reactor was heated and held for one-half hour at 450 C.,during which period the nickel oxide portion of the mixed oxide wasreduced to metallic nickel. The hydrogen flow was then stopped.

The reactor was evacuated to an absolute pressure of approximately 100microns and heated to 975 C. Carbon monoxide evolution was essentiallycomplete after /2 hour at this temperature. Hydrogen was again admittedto the reactor, the pressure being controlled at 50 p.s.i.g. and the gasvelocity adjusted to give about /3 ft./secnd flow through the reducedpellets. Simultaneously, the reactor was cooled to 750 C. The efliuentgas from the reactor was continuously monitored for CO and CH content.After 40 minutes at 750 C., the methane analysis was less than 100p.p.m. and the CO was nil. The reactor was cooled. The product recoveredconsisted of slightly sintered, but friable, metallic pellets. Analysesof the metal indicated that oxygen in excess of that present as thoriawas 1350 p.p.m., carbon was 104 p.p.m., sulfur was 84 p.p.m., surfacearea by nitrogen absorption was 1.2 meter sq./gm., and thoriacrystallite size was 9 millimicrons.

Example A mixture of hydrous oxides was coprecipitated as described inExample 4 and recovered as a filter cake. The cake was dried andsubstantially dehydrated by heating at 450-500 C. for about four hours.The mixed oxide product was charged to a ball mill containing nickelballs approximately A inch in diameter. The mill was rotated for aboutone hour to grind the oxide to a relatively fine powder, 10.5 lbs. ofcarbon black per 100 lbs. of the oxide were added to the mill, andmilling was continued for a total of 24 hours. The oxide-carbon blend,containing 8.55% carbon, was then recovered from the mill.

A one-gram portion of this powder was spread in a thin layer in thebottom of a stainless steel %-inch U tube. Hydrogen containing 2.5%methane was passed through the tube at the rate of 0.1 standard literper minute. The reactor, under 3 p.s.i.g. pressure, was heated to 450 C.in 20 minutes, held for 1 hour at this temperature, then heated in 30minutes to 925 C. and held for 75 minutes at this temperature. Afterthis heating the carbon monoxide content of the efiiuent hydrogen haddropped to 1000 p.p.m. The methane flow was thereupon terminated, butpure hydrogen flow was continued for 30 minutes more, the carbonmonoxide content of the efiiuent thereby dropping to 100 p.p.m.

For decarburization, while maintaining the 0.1 liter per minute flowrate of hydrogen, the hydrogen pressure was elevated to 50 p.s.i.g., andthe temperature was lowered to 850 C. After 170 minutes under theseconditions, the methane concentration leaving the reactor was 140 p.p.m.and the run was terminated. The weight of powder recovered after coolingwas 0.61 gm. Analyses indicated that the metal contained 20 p.p.m.carbon and 2200 p.p.m. excess oxygen. The thoria particle size wasestimated to be 13 millimicrons.

We claim:

1. In a process for making a metal composition containing 0.5 to byWeight of chromium, in which process (1) a mixture comprising (a) anoxide of a metal selected from the group consisting of iron, cobalt andnickel, (b) a particulate refractory oxide having a free energy offormation at 1000 C. greater than 103 kilocalories per gram atom ofoxygen and a particle size below 35 millimicrons, and (c) a chromiumoxide, is prepared and (2) the oxides (a) and (c) are deoxidized byheating the mixture with a carbonaceous reducing agent until the oxygencontent of the product in excess of that combined in oxide (b) is belowabout 2000 p.p.m., the improvement which comprises (3) decarburizing thedeoxidized mixture by heating it at a temperature of 700 to 1000 C. in aflowing gas stream containing hydrogen, the pressure of hydrogen beingat least two atmospheres, said decarburizing being continued at leastuntil substantially all elemental carbon has been removed.

2. A process of claim 1 in which the oxide of (a) is nickel oxide.

3. A process of claim 1 in which the oxide of (a) is cobalt oxide.

4. A process of claim 1 in which the decarburization temperature is inthe range 700 to 800 C.

5. A process of claim 1 in which the decarburization is carried outsubstantially at temperatures in the range of 700 to 800 C. andthereafter is completed at temperatures in the range of 825 to 950 C.,the decarburization being continued until the total carbon content ofthe mixture has been lowered below 1000 p.p.m.

6. A process of claim 1 in which the carbonaceous reducing agent isparticulate carbon and is thoroughly mixed at least with metal oxides(a) and (c), and the deoxidation of the initial oxide mixture is carriedout in vacuum.

7. A process of claim 1 in which the carbonaceous reducing agent is acombination of particulate carbon, in admixture with at least the metaloxides (a) and (c), and a gas stream containing hydrogen and methaneflowing in contact with said admixture, the partial pressures ofhydrogen and methane in said gas stream being related by the expression:

Partial pressure of methane in atmospheres:

F WE where p is the sum of the partial pressures of hydrogen and methanein atmospheres, T is the temperature in the deoxidation Zone in degreesKelvin, and F is a number in the range from 0.73 to 1.2.

8. A process of claim 1 in which the carbonaceous reducing agent is amixture of hydrogen and methane, the partial pressures of which arerelated according to the formula:

Partial pressure of methane in atmospheres:

WOT-1m where p is the sum of the partial pressures of hydrogen andmethane in atmospheres, T is the temperature in the deoxidation zone indegrees Kelvin, and F is a number in the range from 0.73 to 1.2, andduring step (2) the oxides being deoxidized are subjected to penetratingcontact with the hydrogen-methane mixture as a thin layer of powder lessthan about 6 millimeters thick.

References Cited UNITED STATES PATENTS HYLAND BIZOT, Primary Examiner.

W. W. STALLARD, Assistant Examiner.

US. Cl. X.R.

1. IN A PROCESS FOR MAKING A METAL COMPOSITION CONTAINING 0.5 TO 40% BYWEIGHT BY CHROMIUM, IN WHICH PROCESS (1) A MIXTURE COMPRISING (A) ANOXIDE OF A METAL SELECTED FROM THE GROUP CONSISTING OF IRON, COBALT ANDNICKEL, (B) A PARTICULATE REFRACTORY OXIDE HAVING A FREE ENERGY OFFORMATION AT 1000*C. GREATER THAN 103 KILOCALORIES PER GRAM ATOM OFOXYGEN AND A PARTICLE SIZE BELOW 35 MILLIMICRONS, AND (C) A CHROMIUMOXIDE, IS PREPARED AND (2) THE OXIDES (A) AND (C) ARE DEOXIDIZED BYHEATING THE MIXTURE WITH A CARBONACEOUS REDUCING AGENT UNTIL THE OXYGENCONTENT OF THE PRODUCT IN EXCESS OF THAT COMBINED IN OXIDE (B) IS BELOWABOUT 2000 P.P.M., THE IMPROVEMENT WHICH COMPRISES (3) DECARBURIZING THEDEOXIDIZED MIXTURE BY HEATING IT AT A TEMPERATURE OF 700 TO 1000*C. IN AFLOWING GAS STREAM CONTAINING HYDROGEN, THE PRESSURE OF HYDROGEN BEINGAT LEAST TWO ATMOSPHERES, SAID DECARBURIZING BEING CONTINUED AT LEASTUNTIL SUBSTANTIALLY ALL ELEMENTAL CARBON HAS BEEN REMOVED.